Hi Sianne, welcome to S2P.
To upload your LO's, in the post explaining the challenge there should be a pink "Gallery Here". If you click on that it will take you to the right page in the gallery. Then there is an "upload" button on the left hand top area, just below the moving banner (sort of the 3rd row of writing). Click on that, where it says "image" type in the name of your LO and your name (eg funny face - belbaker) that way we know whose it is. In the "description" you can write anything you want us to know about it or even what items you have used etc. Then the "browse" button will let you find the folder on your computer where it is stored. Then hit the "save" button.
Hope this is clear and helps you. If you have any further questions, dont hesitate to ask. Dont worry, it doesn't take long to navigate your way around the site and gallery.
Looking forward to seeing your work.